The development of all-metal (i.e., metals and insulators but no semiconductors) memory known as SpinRAM by Integrated MagnetoElectronics (IME) has addressed three basic challenges at the memory-cell level: (1) scalability (decreasing drive currents and stable error rates with decreasing feature size); (2) high endurance (number of read/write cycles before cell breakdown); and (3) thermal stability of stored information (stability against errors due to thermally-induced transitions between two states that represent different bit values; an effect that increases with decreasing element volume and comes into play at deep nanoscale feature sizes).
Three interrelated features were developed by IME to enable scalability, increased endurance, and thermal stability in a memory array based on SpinRAM memory cells: (1) a closed-flux cell structure; (2) parallel drive lines at the memory cell; and (3) increased film thickness, respectively. These are three of the design features that distinguish SpinRAM cells from other magnetic-RAM designs. Endurance was incorporated in the early SpinRAM cells fabricated. A higher degree of scalability is enabled in the latest SpinRAM design by the fully-closed-flux structure of the memory cells and correspondingly lower drive fields. The issue of thermal stability has been resolved conceptually.
IME has also identified two basic issues beyond the cell level: (1) compatibility of fabrication technology with CMOS processing; at this time, already demonstrated by commercial magnetic RAM; and (2) high capacity. IME separated the development of scalability from that of capacity, as the issues attendant to the two are distinct. IME is pursuing independent programs in parallel to address each issue, with the intent of combining the results at a later development stage.
An important issue relating to magnetic-RAM scalability is control of the demagnetizing field Hd, the field produced by the magnetization M itself.
IME has chosen giant magnetoresistive (GMR) films for memory cell design, despite the smaller signal of some GMR structures relative to that of tunnel magnetoresistance (TMR) structures, for reasons discussed in U.S. Pat. No. 9,741,923 entitled SpinRAM issued on Aug. 22, 2017, the entire disclosure of which is incorporated herein by reference for all purposes.
To realize high capacity, IME implemented two additional development programs. One program involves enhancing GMR by developing a ferromagnetically-coupled GMR superlattice with low drive fields and significantly higher GMR values than previously available, as described in U.S. Pat. No. 8,619,467 entitled High GMR Structure With Low Drive Fields issued on Dec. 31, 2013, the entire disclosure of which is incorporated herein by reference for all purposes. Such structures increase the signal strength of the memory cell. The other program involves development of a three-dimensional structure referred to as 3D SpinRAM.
The functional memory components of SpinRAM—the memory array without support electronics—is made of metal and insulators (no semiconductors), with the potential for monolithic 3D structures (vertically replicated 2D arrays); the storage density per unit area of such a 3D SpinRAM can exceed that of a hard disk; for many mainstream applications, e.g., ones that depend on a specific number of input/output operations per second, it should also cost less than hard disk. To date, SpinRAM has been implemented as a coincident-current architecture of the kind illustrated in FIG. 1 and described, for example, in U.S. Pat. No. 9,741,923, incorporated herein by reference above. In such an architecture, the storage cell is located in the portion of the overlap of the GMR line with the parallel portions of the drive lines, and co-linearity of the drive lines at a given memory cell (as represented by oval 102) ensures that the drive fields at the cell location are co-linear.